Cell based temperature monitoring

ABSTRACT

A system and method for measuring a temperature in at least one energy storage unit. The system includes at least one temperature sensor thermally coupled to the at least one energy storage unit, and a battery management controller in communication with the at least one temperature sensor. The battery management controller is configured to process a temperature of the at least one energy storage unit to obtain an internal temperature in the at least one energy storage unit.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. ProvisionalApplication No. 60/862,532, filed on Oct. 23, 2006, which isincorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to a method and system for monitoringtemperature in a plurality of energy storage units.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a system for managing energy in aplurality of energy storage units connected in series, according toanother embodiment of the present invention;

FIG. 2 is a block diagram of the battery management controller depictedin FIG. 1, according to an embodiment of the present invention;

FIG. 3 is detailed circuit diagram of system for managing energy in aplurality of energy storage units connected in series, according to anembodiment of the present invention;

FIG. 4 is diagram illustrating the charge balancing process in a threecells, according to an embodiment of the present invention;

FIG. 5A is a plot of the voltage across each cell when charge balancingis not enabled or not provided versus the time of charge;

FIG. 5B is a plot of the voltage across each cell when charge balancingis enabled versus the time of charge;

FIG. 6A is a plot of the voltage across each cell during charging whencharge balancing is enabled versus the time of charge;

FIG. 6B is a plot of the voltage across each cell during discharge whencharge balancing is enabled versus the time of discharge;

FIG. 7 is a plot of the voltage across each cell versus the time duringcharge-discharge cycles showing the effect of enabling charge balancingon the voltage across each cell;

FIG. 8 shows an example of implementing temperature determination in acell-pack having a plurality of temperature sensors, according to anembodiment of the present invention;

FIG. 9A shows an example of implementing an internal temperatureestimation method in a cell-pack having a plurality of temperaturesensors, according to one embodiment of the present invention;

FIG. 9B shows another example of implementing an internal temperatureestimation scheme, according to another embodiment of the presentinvention;

FIG. 10 shows an Accelerating Rate Calorimetry experiment on commercialcells showing how cell voltage affects thermal stability;

FIG. 11 is a plot showing the temperature response of each cell in amulticell pack to the heat generated in a simulated short in one cell;

FIG. 12 is a plot of a simulation of temperature versus time for each ofthe cells;

FIG. 13 is a plot of simulated external and internal temperatures versustime for one cell and a plot of external temperature obtained throughone sensor on a pack of cells with a simulation of a short circuit; and

FIG. 14 is a plot of simulated internal and external temperatures of 2cells and a temperature of a pack versus time.

DETAILED DESCRIPTION OF THE SEVERAL EMBODIMENTS

FIG. 1 is a circuit diagram of a system 800 for managing energy andmonitoring temperature in a plurality of energy storage units 802connected in series, according to an embodiment of the presentinvention. The system 800 for managing energy includes a batterymanagement controller 804 and cell balancing circuits 806. The batterymanagement controller 804 is connected to and is configured to controlthe cell balancing circuits 806. The cell balancing circuits 806 areconnected to the energy storage units 802. The battery managementcontroller 804 provides measurement and monitoring of current andvoltages on each of the energy storage units (cells) 802. In fact, thecurrent flowing through the pack and individual cell voltages can bemeasured simultaneously to be able to accurately measure cell impedance.The battery management controller 804 may include cell measurement andcontrol units (shown in FIG. 2), each cell measurement and control unitbeing associated with one of the plurality of energy storage units(cells) 802. The system 800 further comprises one or more temperaturecircuits 808 for connecting to one or more temperature sensors 809. Theone or more temperature sensors 809 measure the temperature of each ofthe plurality of energy storage units or cells individually. In FIG. 1,only one temperature measurement circuit is shown for clarity purposes.However, it must be appreciated that one, two or more temperaturemeasurement circuits (for example, 4 circuits) may be used. Thebalancing circuits 806 are configured to balance charge between theplurality of energy storage units 802. Balancing with system 800 canoccur full time, e.g., during charging, during discharging and duringidle. Balancing occurs cell-to-cell and globally across the pack ofcells.

The battery management controller 804 can be an integrated circuit, aplurality of separate integrated circuits, modules of discretecomponents, or hybrids thereof. FIG. 2 is a block diagram of the batterymanagement controller 804, according to an embodiment of the presentinvention. FIG. 2 shows a single chip implementation of the batterymanagement controller 804. For example the battery management controllercan be implemented using flash based RISC architecture. However, it mustbe appreciated that the battery management controller 804 can beimplemented in a number of configurations by separating one or more ofthe functions that the management controller performs. The managementcontroller 804 comprises a number of sections each of which performs adesired control function. In this embodiment, the battery managementcontroller 804 comprises a RISC CPU 900, first safety FET control 902,second safety 904, LED control 906, coulomb counter 908. The RISC CPU900 is in communication with a flash memory 901 and random access memorySRAM 903. The first and second safety control blocks 902, 904 detectexcessive external current flow and open the pack disconnect FETs (Q3,Q5in FIG. 3). Led Control 906 drives a LED indication of State of Charge(e.g., LEDs 1-5 in FIG. 3). The Coulomb Counter 908 monitors the voltageon a current sense resistor and integrates the current flow to givepassed charge in coulombs. The battery management controller 804 alsoincludes a plurality of cell measurement and control units 910A, 910B,910C and 910D. Each cell management and control unit is associated withone of the plurality of cells 802. Each cell management and control unit910A, 910B, 910C and 910D is capable of measuring the voltage of eachcell 802, measuring the temperature in each cell 802, or a combinationof any thereof, as well as provide charge balancing between a cell andneighboring cells by controlling the cell balancing circuits 806. Adetailed description of a balancing circuit for balancing charge betweencells is provided in U.S. patent application Ser. No. 10/478,757entitled “Method and Apparatus for Managing Energy in Plural EnergyStorage Units,” the contents of which are incorporated herein byreference.

In one embodiment, the balancing circuit 806 includes a reactiveelement, such as an inductor, and a first switch and a second switch.Alternatively, the reactive element can be a capacitor. In a closedposition, the first switch places the balancing circuit in parallel witha first energy storage unit, for example cell 1. In the closed position,the second switch places the balancing circuit in parallel with a secondenergy storage unit, for example cell 2. The switch can be a fieldeffect transistor (FET), however, any suitable switch can be used. TheFETs may include an internal parallel body diode which can be used incell balancing. The measurement and control units 910A, 910B, 910C and910D can operate the FET switch in the balancing circuit to charge theinductor from a first energy storage unit. The diode automaticallyconducts to a second energy storage unit, e.g., a destination energystorage unit, when the switch is turned off.

The battery management controller 804 can control the operation of thebalancing circuit 806, e.g., operate the first and second switches toplace the balancing circuit in parallel with a selected cell, to balancethe charge between cells. In one embodiment, the battery managementcontroller 804 can, for example, support two balancing modes for theenergy storage unit. In a first mode, excess energy within one cellduring charging is burned off through a resistive burn-off process by,for example, the use of a resistive load. In another balancing mode,designated reactive pumping, excess energy from one cell is transferredto a second cell via the balancing circuitry during both charging anddischarging.

The controller 804 further includes an internal oscillator 912, an LDO914, a reset logic circuit 916 in communication with a watchdog circuit917, an SMBus 918 and a power LAN 919. The internal oscillator providesthe main clock for the CPU and an accurate time base for coulombcounting. The Low drop out Voltage (LDO) regulator 914 generates a 2.5Vsupply for the CPU from a higher cell voltage. The SMBus interface isused to communicate with a host appliance (e.g., a Notebook PC). ThePowerLAN interface 919 communicates with each of the cell Measurementand Control function blocks which may be isolated from the main CPU. Thecontroller 804 also includes an internal temperature measurement circuit920. The controller 804 may be provided with a two on-chip temperaturesensors. In addition, to the on-chip temperature sensors, off-chip celltemperature sensors may be provided. The off-chip cell temperaturesensor may use diodes. Temperature sensing can be implemented to measurethe temperature of each cell individually as well as the temperature ofthe cells in the battery pack as whole.

The controller 804 can provide synchronous measurements of voltage andcurrent for each individual cell. Hence, true impedance measurement canbe achieved. In addition, the controller 804 can detect the rate ofcharge change and can keep track of historical data including cell packsignature analysis. Furthermore, the controller 804 achieves full timeeffective cell charge balancing. Cell charge balancing prevents cellabuse, assures proper cell charging, and maximizes useable cycle lifeand runtime of the cell pack. These features allow improved monitoringof possible cell anomalies and/or other events that may occur during theusage lifetime of the cells.

FIG. 3 is a detailed circuit diagram of system 1000 for managing energyand monitoring temperature in a plurality of energy storage unitsconnected in series 1002, according to an embodiment of the presentinvention. The management system 1000 includes a battery managementcontroller 1004. In one embodiment, the battery management controller1004 is the battery management controller 804 described above. Themanagement system includes temperature measurement diodes 1006A, 1006B,1006C and 1006D. These diodes are used to measure the temperature on asurface of each cell via a thermal conductor strip disposed, forexample, between adjacent pairs of cells. In this embodiment, a pair ofdiodes is used to measure the temperatures of each cell to improvetemperature coefficient (mV per deg. C.). Alternatively, one diode mayalso be used to measure the temperature on a surface of each cell. Themanagement system further includes balancing circuits 1008A, 1008B and1008C. As shown in FIG. 3, balancing circuits 1008A, 1008B and 1008Ccomprise inductors L1, L2 and L3, respectively. The management systemmay be integrated in a single circuit, thus allowing to reduce thenumber of components used and thus ultimately reduce the cost offabrication.

FIG. 4 is diagram illustrating the charge balancing process in threecells, according to an embodiment of the present invention. Althoughthree cells are depicted herein, it must be appreciated that any numberof cells can be contemplated and balanced using the charge balancingprocess. For example, as shown in FIG. 1, four cells 802 are connectedin series. In FIG. 4, three cells 802 are shown connected in series. Thecells 802 are provided with two thresholds. An upper thresholdrepresenting the over voltage cut off, i.e., the voltage at which thecharge of the cells is halted. A lower threshold representing the undervoltage pack cutoff, i.e., the voltage at which the charge of the cellsis started. Charge balancing circuits 806 labeled in FIG. 4 “power pump”manages the charge transfer, between the three cells 802. The chargebalancing process enables the charge among cells to be balanced. Thebalancing of charge is controlled by controller 804, shown in moredetail in FIGS. 1, 2 and 3. The balancing may occur during the charge ofthe cells, during the discharge of the cells and/or when the cells areidle. The balancing can occur cell-to-cell and/or globally across theplurality of cells. The balancing process enables cell over-voltageabuse to be minimized and each cell's charge imbalance history to betracked. Furthermore, the balancing process enables early failure modesdetection including change in self discharge or capacity divergence.Thus, the balancing process improves the overall reliability, safety andefficiency of the cells in the cell pack. In addition, the balancingprocess improves the cell pack life cycle.

FIG. 5A is a typical plot of the voltage across each cell during acharging cycle when charge balancing is not enabled or not providedversus the time of charge. As shown in FIG. 5A, the third cell (cell 3)in a four-cell pack reaches the 4300 mV (4.3 V) safety limit (at about8000 s) before the other cells (i.e., cell 1, cell 2 and cell 4) reachtheir full charge limit of 4200 mV. As a result, the charging cycle isended (at about 8000 s) before the other cells (cell 1, cell 2 and cell4) reach their full charge. This pattern would be repeated on subsequentcharge cycles abusing cell 3 and also limiting the full charge capacity.

FIG. 5B is a plot of a simulation of a voltage across each cell during acharging cycle when charge balancing is enabled versus the time ofcharge. The balancing of the charge among the cells (cell 1, cell 2,cell 3 and cell 4) permits the charging to complete without abusing anycell. The cells are balanced within 10 mV, and all reach about 4200 mV,after approximately 10000 seconds. For example, as shown in FIG. 5B, thecharge in cell 3 is reduced by transferring some of the charge in cell 3to the other cells (cell 1, cell 2 and/or cell 4).

FIG. 6A is a typical plot of voltage across each cell during chargingwhen charge balancing is enabled and shows the complete charge phase.FIG. 6B is plot of a corresponding discharge phase when charge balancingis enabled versus the time. The pack current through the seriesconnected cells is also plotted versus time of charge and discharge. Asshown in FIGS. 6A and 6B a tight charge control is established betweenthe various cells (cell 1, cell 2, cell 3 and cell 4) during both thecharging phase (FIG. 6A) and the discharging phase (FIG. 6B). This isreflected by the very small, voltage difference between the voltagesacross the cells during the charge and discharge phases. The charge anddischarge profiles show minimum stress for the cells. A precise controlof the charge and discharge process is maintained throughout the lifecycle of the cells. As a result, a maximum runtime using the cells andlonger cell-pack can be achieved.

FIG. 7 is a typical plot of voltage across each cell in a sevencell-pack versus the time showing the effect of enabling chargebalancing on the voltage across each cell. Specifically, it can beobserved from this plot that when charge balancing is not enabled, thevoltage across each cell is different. This is can be due, for example,to variance in state of charge, impedance and/or capacity of the cells.However, when charge balancing is enabled, as indicated by the label “A”in FIG. 7, the voltage across each of the cells equalizes. The cellcharge balance can be restored, for example, within approximately 40minutes from enabling of the charge balancing as indicated by the label“B” in this plot.

Returning to FIGS. 1, 2 and 3, in addition to controlling the balancingof the cells using cell balancing circuits 806, the controller 804 alsoincludes cell temperature measurement/monitoring circuits 910A, 910B,910C and 910D. Temperature sensing/monitoring in each individual cellcan be implemented to measure/monitor the temperature of each cellindividually as well as the temperature of the battery pack as whole.The controller 804 can be configured to process a temperature at asurface of each individual cell to obtain an internal temperature ineach of the cells. The goal of cell based temperature monitoring can beto determine an accurate absolute temperature at each cell to ensure asafe operation range and also to sense thermal signatures which indicatea heat (Q) dump from a micro-short circuit in the cell. In order toaccomplish the above goals, one approach is to eliminate effects ofother heat sinks or sources at the temperature sensors. The heat sinksor heat sources can be eliminated physically by insulating or shieldingthe cell (or cells) or mathematically by measuring heat flow using twosensors and performing an extrapolation. Another approach is to apply apre-emphasis procedure to determine the internal temperature of the cellusing either a measured surface temperature or a calculated extrapolatedsurface temperature. There are various implementations to accomplish theabove cell based temperature monitoring. The following paragraphsdescribe some of the implementations.

FIG. 8 shows an example of a cell-pack having a plurality of temperaturesensors, according to an embodiment of the present invention. In thisembodiment, the effects from heat sinks or heat sources unrelated to thecells are eliminated physically by shielding the sensor. For example,the sensor is thermally isolated by the cells. The cell pack 2000includes 4 juxtaposed pairs of cells 2003, i.e., a total of 8 cells2003. A plurality of temperature sensors 2001, in this case 4temperature sensors 2001, are placed between the pairs of cells 2003 incontact with a surface of each cell 2003. The plurality of temperaturesensors 2001 are connected to controller 804, 1004. Specifically, onetemperature sensor (e.g., diodes) is placed in the valley between eachof the juxtaposed pairs of cells 2003 and in good thermal contact withthe cells. The sensor is insulated from external temperatures by aninsulating air pocket. Each one of the temperature sensors measures themean temperature of the two cells. The two cells are connected inparallel. Therefore, the two cells act as a single cell. The cell pack2000 also includes electronic circuitry 2002 for temperature monitoringand cell charge management including cell charge balancing. Thetemperature sensors 2001 are connected to the electronic circuitry 2002through a flexible circuit (not shown). Using a flexible circuit allowsto place the electronic circuitry 2002 away from the temperature sensors2001. Hence, the temperature sensors 2001 are “thermally isolated” fromoutside heat sources, including heat that may be generated by theelectronic circuitry 2002. In this way, the temperature sensors 2001measure a temperature T_(meas) of the cells 2003 with minimal orsubstantially no perturbation from outside heat sources including heatgenerated by the electronic circuitry 2002. In other words, the cells2003 and associated temperature sensors 2001 are shielded from outsideheat sources. In this case, a temperature inside each of the cells canbe determined by compensating for the thermal mass, i.e., thermal lag,of the temperature sensors by employing pre-emphasis based on the rateof change of temperature from the temperature sensors. In thisembodiment, the cells 2003 have a cylindrical shape and the temperaturesensors 2001 are disposed between each pair of cells 2003. However, thecells 2003 can have other shapes, such as for example, a parallelepipedshape, in which case the temperature sensors 2001 can be disposed incontact with flat surfaces of two superposed cells 2003.

In one embodiment of the invention, compensation for thermal lagconstant τ can be obtained by adding a term (derivative.τ) to themeasured or calculated surface temperature. To limit the noise in thisterm, a first order filter of time constant τ/DL (where DL is thederivative lift) is applied prior to calculating the derivative. Afilter, as expressed by equation (2), reduces or dampens thecontribution of the change in the measured temperature T_(meas) bymultiplying by a factor β. Hence, the estimate of internal celltemperature or compensated temperature T_(comp) in this case iscalculated by the controller 804 as follows:

$\begin{matrix}{{T_{comp} = {T_{meas} + {\left( {T_{lowpass}^{\prime} - T_{lowpass}^{\prime}} \right){\tau/{dt}}}}},{where}} & (1) \\{{T_{lowpass} = {{T_{lowpass}^{\prime}\left( {1 - \beta} \right)} + {\beta \; T_{meas}}}}{with}} & (2) \\{\beta = \frac{{dt} \cdot {DL}}{\tau}} & (3)\end{matrix}$

and T_(meas) being the measured or calculated surface temperature of thecell, dt being the sample time and T_(lowpass)′ corresponding to aprevious sample of T_(lowpass).

The terms T_(lowpass) and T_(lowpass′) are initialized to the measuredtemperature T_(meas).

The thermal lag constant τ is fixed for a given layout and can becharacterized using a measurement test. In the measurement test, heat isapplied at a certain time and the response of the temperature sensor isrecorded and the time lag between the application of the heat and theresponse time of the temperature sensor is recorded. The response willapproximate to a 1^(st) order exponential response. The thermal lagconstant τ can be found from a best fit exponential response.

In order to improve the accuracy of an absolute temperaturedetermination per cell in the case where the heat sinks or heat sourcesare not completely eliminated physically, the effects of the heat sourcecan be taken into account and eliminated mathematically by measuringheat flow using two sensors and performing an extrapolation. Thecontroller 804 can be configured to perform the temperatureextrapolation method. The temperature extrapolation method can be usedwhen it is desired to minimize or substantially eliminate effects of“external” heat sources on the measurement of a cell temperature. Theexternal heat sources may include heat sources from electronic devicesor the like. In minimizing the effects of external heat sources on atemperature of a cell, temperature gradients originating from amicro-short in the cell can be distinguished from external beat sourceand measured.

FIG. 9A depicts an implementation of a temperature sensing scheme,according to an embodiment of the present invention. In this embodiment,the effects of external heat sinks or heat sources are minimized byplacing thermally conductive strip 1304 between cells 1301 and 1302. Tofurther improve measurement accuracy, a heat flow from a potentialexternal heat source is measured using a pair of temperature sensors anda temperature of the surface of the cell is extrapolated. In a pack ofcells comprising a plurality of cells 1300, a first temperature sensor1303 is connected to a thermally conducting strip 1304. The strip 1304is disposed between two adjacent cells 1301 and 1302 in the plurality ofcells 1300. The cells 1301 and 1302 are connected in parallel and act asa single cell. The strip 1304 can include a metal such as copper, nickelor the like. A thermal impedance of the strip 1304 and its interface tothe cells is relatively small. The first temperature sensor 1303 isconnected to a second temperature sensor 1305 via a material 1306 havinga certain thermal impedance. The dominant path for heat flow to/fromsensor 1303 is via this material and this heat flow is monitored usingthe two temperature sensors 1303 and 1305. The temperature at a point1308 between the cells 1301 and 1302 is extrapolated from the externaltemperature measurements using the two temperature sensors 1303 and1305.

FIG. 9B depicts another implementation of a temperature sensing scheme,according to an embodiment of the present invention. In this embodiment,similar to the embodiment depicted in FIG. 9A, a heat flow from apotential external heat source is measured using a pair of temperaturesensors S1 and S2 and a temperature of the cell extrapolated. However,in FIG. 9B, the temperature sensing element is not physically locatedbetween cells. In a temperature sensing apparatus 1310, a firsttemperature sensor 1313 is connected to an electrical terminal 1318 of acell 1311 via strip 1314. The strip 1314 can be electrically conductiveor nonconductive. In one embodiment, the strip 1314 can include a metalsuch as copper, nickel or the like. In the case where the strip 1314 isboth electrically conductive and thermally conductive, the strip 1314can be used to electrically and thermally connect a pole of the cell toone terminal of sensor 1313. Similar to the above embodiment, a thermalimpedance of the strip 1314 is is fixed for a given material andgeometry. The first temperature sensor 1313 is connected to a secondtemperature sensor 1315 via a material 1316 having a fixed thermalimpedance and represents the dominant heat flow path from one or morepotential external heat sources. Heat flow is monitored using the twotemperature sensors 1313 and 1315. The temperature at a point 1318 isextrapolated from the external temperature measurements using the twotemperature sensors 1313 and 1315.

Referring to both FIGS. 9A and 9B, two temperatures T₁ and T₂ aremeasured using the two temperature sensors 1303, 1313 and 1305, 1315,respectively. T₁ is the temperature measured by the sensor 1303, 1313which is closest to the cells 1301 and 1302 or cell 1311. T₂ is thetemperature measured by the temperature sensor 1305, 1315 which isfarthest to the cells 1301 and 1302 or cell 1311. The material 1306,1316 separating the two temperatures sensors 1303 and 1305 ortemperature sensors 1313 and 1315 has a thermal impedance Rb. The strip1304 disposed (e.g., inserted) between the two cells 1301 and 1302 has athermal impedance Ra. The strip 1314 connected to a terminal of the cell1322 has a thermal impedance Ra. The two thermal impedances Ra and Rb,determine an extrapolation ratio α defined by the following equation,

$\begin{matrix}{\alpha = \frac{{Ra} + {Rb}}{Rb}} & (4)\end{matrix}$

α is fixed for a given layout and can be characterized using ameasurement test. The controller 804 then calculates the extrapolatedtemperature T_(ext) at point 1308 between the cells 1301 and 1302 or atpoint 1318 at cell 1311 using the extrapolation ratio α and the twotemperatures T₁ and T₂ measured by the temperature sensors 1303, 1313and 1305, 1315 as follows:

T _(ext) =αT ₁+(1−α)T ₂  (5)

Similar to the description above, the controller 804 calculates atemperature inside each cell by using the extrapolated surfacetemperature T_(ext) at point 1308 between cells 1301 and 1302 and atpoint 1318 at cell 1311 and by compensating the thermal mass, i.e.,thermal lag, of the temperature sensors by employing pre-emphasis basedon the rate of change of temperature from the temperature sensors.

In this case, the estimate of internal cell temperature or compensatedtemperature T_(comp) can be calculated using the extrapolatedtemperature T_(ext), as follows:

T _(comp) =T _(ext)+(T _(lowpass) −T _(lowpass)′)τ/dt,  (6)

where

T _(lowpass) =T _(lowpass)′(1−β)+βT _(ext)  (7)

and dt being the sample time, β being expressed in equation (3), andT_(lowpass)′ corresponding to a previous sample of T_(lowpass).

By estimating the internal temperature of the cell, the effect ofambient temperature or surrounding temperature change on the cell isreduced, thus providing a more accurate measurement of the temperatureof the cell. In addition, by estimating the internal temperature, thewarm-up delays due to thermal mass can be minimized, hence providing afaster temperature response. The internal temperature estimation methodallows the tracking or monitoring of the internal temperature in eachcell and thus provide a temperature history for each cell continuously.In addition, the internal temperature estimation method as related tothe arrangement in FIGS. 9A and 9B allows for active “noisecancellation” in temperature measurements by extrapolating from thetemperature of two sensors positioned appropriately and separated with amaterial having a certain thermal impedance to a point of interestinside the cell. As a result, this method provides estimates of internaltemperature which are sufficiently free of external influences andsensor delay to allow detection of small but sudden heat generationwithin the cell which provides an early warning sign of cell failures.In addition to cell temperature monitoring, the internal temperatureestimation method can also be used for temperature monitoring ofprotection devices such as FETs and fuses. Multiple sensors may also beused to acquire a temperature measurement. This provides redundantmeasurements which increases reliability.

FIG. 10 shows a graph of an Accelerating Rate Calorimetry experiment oncommercial cells showing how cell voltage affects thermal stability. Thetemperature measurement is performed on commercial cells having LiCoO₂cathodes. This graph shows how cell voltage affects thermal stability.It can be seen that rate of exothermic heat generation increases withboth temperature and voltage. The onset of thermal runaway occurs whenthe rate of beat generation exceeds the rate of heat loss of the cell toits surroundings. A cell's cooling rate depends on temperature ofneighboring cells. As a result, the temperature of all cells globallyplays a role in the thermal stability of each cell.

FIG. 11 is a plot of the temperature for each cell in a four cell packversus time during a test when one cell is exposed to an external heatsource. The test is performed by applying a heat source to cell 1 and“measuring” the temperatures at all cells (cells 1, 2, 3 and 4) As shownin this graph, the measured temperature in cell 1 experiences a 7 deg.C. rise in about 20 minutes while a temperature sensor placed on thecell pack as a whole, which corresponds to the temperature sensor placedon cell 3, measures a slower and smaller temperature rise of about 2.5deg. C. in about 30 minutes. Hence, the temperature measured for thepack of cells as a whole. is not a good indication of the thermal statusof each cell. In addition, this plot also shows that heat in one cell(cell 1) may be transmitted to other neighboring cells (cell 2, cell 3and cell 4). As a distance between a heat source (for example one cellgenerating heat) and other cells increases, the effects of beat on theother cells diminishes. Therefore, monitoring the temperature of eachcell may be the best approach for detecting and thus preventingoverheating of each cells.

FIG. 12 is a plot of temperature versus time for each of the cellsduring a test to simulate a soft short in one cell. A sharp raise intemperature is simulated in one cell (cell 1) and the temperatureeffects are “measured” (in the simulation) in each cell in the cell-pack(cell 1, cell 2, cell 3 and cell 4). As shown by this plot, the singletemperature monitoring of one cell (cell 3) fails to capture or detectthe rapid rise temperature that occurred on cell 1. Hence, temperaturemonitoring of each cell can enable events such as rapid temperature riseon any one of the cells to be captured. The rate of change intemperature is one parameter that may signal abnormal events such as arapid rise in temperature. The rate of change in temperature can bedefined as a slope of temperature as function of time. For example, bysetting a test threshold above the normal rate of temperature change ina cell, the controller 804 can compare rates of change of temperature incells to the threshold and a course of action can be taken depending onthe outcome. For example, a normal rate of change of temperature of acertain type of cell can be characterized during a charge/dischargecycle and used to determine a suitable exception threshold in thecontroller 804. The controller 804 can then compare the rate of changeof temperature of a cell to the normal rate of temperature change. Whenthe rate of change is greater than the normal rate of change, thissignals a rapid raise of temperature and consequently, the controllercan be configured to take appropriate action. When, on the other hand,the rate of change is smaller or equal to the normal rate of change, thecontroller 804 can be configured to not take any action since the cellis within the normal established temperature conditions and hencerunning in a “normal” operation condition.

In addition, the controller 804 can be configured to compare the rate ofchange of estimated internal temperature to a safety rate of changethreshold, as described above with regard to FIG. 12, to determine whenthe corresponding cell is malfunctioning. Depending on the outcome, thecontroller 804 can take appropriate action, such as warning the user ofthe cell pack of an abnormal event and/or discharging the cells in thecell pack.

FIG. 13 is a plot of simulated external and internal temperatures versustime for one cell and a plot of external temperature obtained with onesensor on a pack of cells with a simulation of a short circuit. A shortcircuit is simulated in one cell (cell 1) and the internal simulatedtemperature profile in the cell 1 is compared with the externaltemperature profile of cell 1. The external temperature on the one cellis “measured” (in the simulation) using a temperature sensor (one ormore temperature sensors) disposed, for example, on an external surfaceof the cell. The single pack temperature sensor is usually placed insidethe enclosure that houses the pack of cells. The single pack temperaturesensor is not necessarily placed in the vicinity or on a surface of acell. In general, the plot in FIG. 13 shows that on the one cell (cell1), the internal temperature exhibits a rapid rise to a temperature ofabout 29 deg. C. almost immediately, when the short circuit occurs,while the external temperature “measured” with a sensor shows a delayedrise i.e. slow signature, with a peak temperature of 27 deg. C. reachedwithin approximately 2 minutes. The delayed rise in temperature is dueto thermal mass, i.e., thermal lag, of the temperature sensors. On theother hand the single pack sensor, which measures a temperature of thecells pack as a whole, shows a slow temperature rise and low temperaturenot exceeding 22 deg. C. even after about 10 minutes, thus failing todetect the short circuit event. This result further demonstrates theneed for measuring the temperature of the cells individually as opposedto measuring the temperature of the cells pack as whole.

FIG. 14 is a plot of simulated internal and, external temperatures of 2cells and a temperature of a cells pack captured using a single packtemperature sensor versus time. This plot shows that the internaltemperature measurement employing an internal temperature estimationmethod provides the fastest response time when compared with externaltemperature measurement. Indeed, the external temperature responseexhibits a temperature response delay, as shown by the double arrow inFIG. 14. While the external temperature measurement on one cell exhibitsa delay in the response to a temperature rise, the single packmeasurement sensor does not even detect the temperature rise event thatoccurred in the one cell. This demonstrates that the internaltemperature estimation method is most appropriate in detecting earlierevents that may occur in a cell such as a rising temperature due to ashort circuit or the like in one or more cells. As stated previously, inthe internal temperature estimation method, a pre-emphasis procedure isused to calculate the internal temperature using an external temperaturemeasured on a surface of the cell. The internal temperature estimationmethod compensates for heat loss and response time of the temperaturesensor disposed on a surface of the cell.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample, and not limitation. It will be apparent to persons skilled inthe relevant art(s) that various changes in form and detail can be madetherein without departing from the spirit and scope of the presentinvention. In fact, after reading the above description, it will beapparent to one skilled in the relevant art(s) how to implement theinvention in alternative embodiments. Thus, the present invention shouldnot be limited by any of the above-described exemplary embodiments.

Moreover, the method and apparatus of the present invention, likerelated apparatus and methods used in the electronics arts are complexin nature, are often best practiced by empirically determining theappropriate values of the operating parameters, or by conductingcomputer simulations to arrive at best design for a given application.Accordingly, all suitable modifications, combinations and equivalentsshould be considered as falling within the spirit and scope of theinvention.

In addition, it should be understood that the figures, are presented forexample purposes only. The architecture of the present invention issufficiently flexible and configurable, such that it may be utilized inways other than that shown in the accompanying figures.

Further, the purpose of the Abstract of the Disclosure is to enable theU.S. Patent and Trademark Office and the public generally, andespecially the scientists, engineers and practitioners in the art whoare not familiar with patent or legal terms or phraseology, to determinequickly from a cursory inspection the nature and essence of thetechnical disclosure of the application. The Abstract of the Disclosureis not intended to be limiting as to the scope of the present inventionin any way.

1. A system for measuring a temperature in at least one energy storageunit, comprising: at least one temperature sensor thermally coupled tothe at least one energy storage unit; and a battery managementcontroller in communication with the at least one temperature sensor,wherein the battery management controller is configured to process atemperature of the at least one energy storage unit to obtain aninternal temperature of the at least one energy storage unit.
 2. Thesystem of claim 1, wherein the internal temperature of the at least oneenergy storage unit is computed by taking into account a thermal lag ofthe at least one temperature sensor.
 3. The system of claim 2, whereinthe internal temperature T_(comp) in the at least one energy storageunit is calculated using a temperature T_(meas) measured by the at leastone temperature sensor.
 4. The system of claim 3, wherein the internaltemperature T_(comp) in the at least one energy storage unit isemploying a derivative term and a time constant τ.
 5. The system ofclaim 4, wherein the internal temperature T_(comp) is calculated asfollows:T_(comp) = T_(meas) + (T_(lowpass) − T_(lowpass)^(′))τ/dt, whereT_(lowpass) = T_(lowpass)^(′)(1 − β) + β T_(meas) with$\beta = \frac{{dt} \cdot {DL}}{\tau}$ and dt being the sample time andT_(lowpass)′ corresponding to a previous sample of T_(lowpass), τ is atime constant and DL is the derivative lift.
 6. The system of claim 1,wherein the at least one temperature sensor includes a first temperaturesensor, a second temperature sensor and a material is disposed betweenthe first temperature sensor and the second temperature sensor.
 7. Thesystem of claim 6, further comprising a strip, the strip being connectedto the first temperature sensor and disposed proximate the at least oneenergy storage unit.
 8. The system of claim 7, wherein the strip isdisposed between two adjacent storage units.
 9. The system of claim 7,wherein the battery management controller is further configured toextrapolate a temperature at a point between two energy storage unitsfrom external temperature measurements using the first temperaturesensor and the second temperature sensor.
 10. The system of claim 9,wherein the extrapolated temperature T_(ext) between the two energystorage units is dependent on an extrapolation ratio and a temperaturemeasured by the first temperature sensor and a temperature measured bythe second temperature sensor.
 11. The system of claim 10, whereinT_(ext)=aT₁+(1−a)T₂, where a is the extrapolation ratio and T₁ and T₂are temperatures measured by the first temperature sensor and the secondtemperature sensor, respectively.
 12. The system of claim 11, whereinthe extrapolation ratio ${a = \frac{{Ra} + {Rb}}{Rb}},$ where Ra is athermal impedance of the strip and Rb is a thermal impedance of thematerial disposed between the first temperature sensor and the secondtemperature sensor.
 13. The system of claim 9, wherein the batterymanagement controller is further configured to determine a correctionfactor that compensates for thermal lag to the first and the secondtemperature sensors and determine the internal temperature using thecorrection factor and the extrapolated temperature T_(ext).
 14. Thesystem of claim 13, wherein the internal temperature T_(comp) in the twoenergy storage units is determined from a derivative term and a timeconstant τ.
 15. The system of claim 14, wherein the internal temperatureT_(comp) is calculated as follows:T_(comp) = T_(text) + (T_(lowpass) − T_(lowpass)^(′))τ/dt, whereT_(lowpass) = T_(lowpass)^(′)(1 − β) + β T_(ext) with$\beta = \frac{{dt} \cdot {DL}}{\tau}$
 16. (canceled)
 17. (canceled) 18.(canceled)
 19. (canceled)
 20. (canceled)
 21. (canceled)
 22. (canceled)23. (canceled)
 24. (canceled)
 25. (canceled)
 26. (canceled) 27.(canceled)
 28. (canceled)
 29. (canceled)
 30. (canceled)
 31. (canceled)32. (canceled)
 33. (canceled)
 34. (canceled)
 35. (canceled) 36.(canceled)
 37. A system for estimating a temperature in at least twoenergy storage units, comprising: at least one temperature sensorthermally coupled to the at least two energy storage units; and abattery management controller in communication with the at least onetemperature sensor, wherein the at least one temperature sensor issubstantially thermally isolated by the at least two energy storageunits.
 38. The system of claim 37, wherein the battery managementcontroller is configured to process the surface temperature of the atleast two energy storage units to obtain an internal temperature in eachof the at least two energy storage units.
 39. The system of claim 38,wherein the internal temperature in the at least two energy storageunits is computed by taking into account a thermal lag of the at leastone temperature sensor.
 40. The system of claim 39, wherein the internaltemperature T_(comp) in the at least two energy storage units iscalculated using a temperature T_(meas) measured by the at least onetemperature sensor.
 41. The system of claim 40, wherein the internaltemperature T_(comp) in the at least two energy storage units iscalculated by multiplying a derivative term by a time constant τ. 42.The system of claim 41, wherein the internal temperature T_(comp) iscalculated as follows:T_(comp) = T_(meas) + (T_(lowpass) − T_(lowpass)^(′))τ/dt, whereT_(lowpass) = T_(lowpass)^(′)(1 − β) + β T_(meas) with$\beta = \frac{{dt} \cdot {DL}}{\tau}$ and dt being the sample time andT_(lowpass)′ corresponding to a previous sample of T_(lowpass) τ is atime constant and DL is the derivative lift.